For electronic devices, particularly power devices, silicon carbide offers a number of physical, chemical and electronic advantages. Physically, the material is very hard and has an extremely high melting point, giving it robust physical characteristics. Chemically, silicon carbide is highly resistant to chemical attack and thus offers chemical stability as well as thermal stability. Perhaps most importantly, however, silicon carbide has excellent electronic properties, including high breakdown field, a relatively wide band gap (about 3.0 eV and 3.2 eV at room temperature for the 6H and 4H polytypes respectively), high saturated electron drift velocity, giving it significant advantages with respect to high power operation, high temperature operation, radiation hardness, and absorption and emission of high energy photons in the blue, violet, and ultraviolet regions of the spectrum.
Accordingly, interest in silicon carbide devices has increased rapidly and power devices are one particular area of interest. As used herein, a “power” device is one that is designed and intended for power switching and control or for handling high voltages and/or large currents, or both. Although terms such as “high field” and “high temperature” are relative in nature and often used in somewhat arbitrary fashion, “high field” devices are generally intended to operate in fields of 1 or more megavolts per centimeter, and “high temperature” devices generally refer to those operable above the operating temperatures of silicon devices; e.g, at least about 200° C. and preferably 250°-400° C., or even higher. For power devices, the main concerns include the absolute values of power that the device can (or must) handle, and the limitations on the device's operation that are imposed by the characteristics and reliability of the materials used.
Silicon carbide-based insulated gate devices, particularly oxide-gated devices such as MOSFETs, must, of course, include an insulating material in order to operate as IGFETs. Similarly, MIS capacitors require insulators. By incorporating the insulating material, however, some of the physical and operating characteristics of the device become limited by the characteristics of the insulator rather than by those of silicon carbide. In particular, in silicon carbide MOSFETs and related devices, silicon dioxide (SiO2) provides an excellent insulator with a wide band gap and a favorable interface between the oxide and the silicon carbide semiconductor material. Thus, silicon dioxide is favored as the insulating material in a silicon carbide IGFET. Nevertheless, at high temperatures or high fields or both, at which the silicon carbide could otherwise operate satisfactorily, the silicon dioxide tends to electrically break down; i.e., to develop defects, including traps that can create a current path from the gate metal to the silicon carbide. Stated differently, silicon dioxide becomes unreliable under the application of high electric fields or high temperatures (250°-400° C.) that are applied for relatively long time periods; i.e., years and years. It will be understood, of course, that a reliable semiconductor device should have a statistical probability of operating successfully for tens of thousands of hours.
Additionally, those familiar with the characteristics of semiconductors and the operation of semiconductor devices will recognize that passivation also represents a challenge for structures other than insulated gates. For example, junctions in devices such as mesa and planar diodes (or the Schottky contact in a metal-semiconductor FET) produce high fields that are typically passivated by an oxide layer, even if otherwise non-gated. Such an oxide layer can suffer all of the disadvantages noted above under high field or high temperature operation.
Accordingly, IGFET devices formed in silicon carbide using silicon dioxide as the insulator tend to fall short of the theoretical capacity of the silicon carbide because of the leakage and the potential electrical breakdown of the silicon dioxide portions of the device.
Although other candidate materials are available for the insulator portion of silicon carbide IGFETs, they tend to have their own disadvantages. For example, high dielectrics such as barium strontium titanate or titanium dioxide have dielectric constants that drop dramatically when a field is applied. Other materials have poor quality crystal interfaces with silicon carbide and thus create as many problems (e.g., traps and leakage current) as might solved by their high dielectric constant. Others such as tantalum pentoxide (Ta2O5) and titanium dioxide (TiO2) tend to exhibit an undesired amount of leakage current at higher temperatures. Thus, simply substituting other dielectrics for silicon dioxide presents an entirely new range of problems and disadvantages in their own right.
Recent attempts to address the problem have included the techniques described in U.S. Pat. No. 5,763,905 to Harris, “Semiconductor Device Having a Passivation Layer.” Harris '905 appears to be somewhat predictive, however, and fails to report any device results based on the disclosed structures.
Similarly, Metal-Insulator-Metal (MIM) capacitors on wide bandgap Monolithic Microwave Integrated Circuits (MMICs) may be subject to high voltages at elevated temperatures. Accordingly, such capacitors typically are desired to have a mean time to failure (MTTF) of 107 for a stress condition of, for example, as high as 200 volts at temperatures of up to about 300° C. Unfortunately, these extreme fields and temperatures may cause a conventional silicon nitride MIM capacitor to suffer from excessive leakage current and/or poor reliability (e.g. MTTF of about 200 hours).
Therefore, the need exists for a dielectric composition or structure that can reliably withstand high electric fields while minimizing or eliminating current leakage, and while operating at high temperatures.